The clarity filter can be found in the sharpen category of the Affinity Photo's filter menu commands. The effect is also available via the layer menu and new live filter so can be used as a non destructive effect as well.
It is great for localized contrasts applied to an image or photo. Best for mid tones and creates an interesting intense sharpen effect and highlights the outlines / edges (details) of an image.
The radius setting controls the strength of the effect so a high setting you can see the effect far more pronounced, low setting, you will hardly notice it. You can apply the sharpening effect multiple times to an image but repeat applies will end up with an image looking very strange with high contrasts in parts of the image and low elsewhere.
You can also use the sharpening features with the layer menu and fade command so the effect is faded using opacity / fade amount as well as the blending mode used.
It is of no use with text and shapes etc though you can use it on them. If you want to use it with text / shapes then the best solution is to merge them all down to a single layer or add effects to the text and shape such as blurs and then use the tool with that.
You can also use the effect with selections so you don't have to apply the effect to the entire image, you can use the select menu to select the mid tones etc and then just apply the effect to that area.
You can also use the effect with color channels by going to the view menu and studio and selecting the composite red etc.
As with ll other effects, you can use this in macros so the setting can be stored and reapplied at any point in the future.
You can repeat the command multiple times and it does create quite a pronounced effect with the repeat applies though perhaps not visually appealing. You can apply it multiple times and also use with the original by using blending modes in the layers panel and also combining it with blurring effects such as Gaussian and Motion to create interesting sketch and charcoal etc effects. Repeat apply it multiple times with the layer selected, also have another layer and apply the effect multiple times to that (perhaps re-size the layer) and then combine that to create some truly odd designs, Or perhaps use the result of multiple applies and select the resulting area of white and select similar and then invert that selection and copy and paste as a layer and apply layer fx to the that new design to create weird drip[ping 3D effects, all using this effect.